[unreadable] This competitive supplement will allow investigators to complete the clinical trial: Triiodothyronine in Infants and Children after Cardiopulmonary bypass (TRICC). The primary clinical endpoint is time to extubation after cardiopulmonary bypass (CPB). This supplement will expand the program, adding new clinical sites, and continuing others added after the original grant award. Four sites will operate in addition to the parent site, Children's Hospital, Seattle. CPB induces marked and persistent depression of circulating thyroid hormones, contributing to postoperative morbidity. Randomized studies in adults demonstrate that parenteral triiodothyronine (T3) repletion during coronary bypass surgery improves postoperative ventricular function, reduces the need for inotropic agents and mechanical devices, and decreases the incidence of atrial fibrillation. Postoperative contractile dysfunction contributes to morbidity, mortality, and requirement for mechanical support following congenital heart surgery in infants and children. Postoperative cardiac dysfunction can be directly linked to thyroid hormone depression in this population. Despite the positive results obtained in comparable adult populations, no extensive evaluation of thyroid repletion has been completed in infants or children undergoing CPB. This Orphan Product Development funded study will provide essential safety and efficacy data to better define the use of triiodothyronine (Triostat) as a new indication in infants and young children, whom undergo CPB. Development of new inotropic therapies for the very young child has been hampered by the lack of focused randomized studies, as well as relatively modest industry support for this orphan population. This completed and fully recruited study will serve as a model for developing future hormonal and pharmacological interventions in cardiac patients of this age group. The investigator hypothesizes that Triostat prior to CPB and after removal of aortic cross clamp will be safe and will result in significant improvements in clinical outcome parameters and cardiac contractile function Primary Aims: (1) to determine if the safety profile differs between the parenteral Triostat treated group and the placebo group and (2) to determine if clinical outcome and cardiac contractile function differ between the Triostat treated group and the placebo group. [unreadable]